major: Remove features which are not supported by v2 components

Remove features which are not supported by v2 components (#177)

- Requires node 12 or later.
- Removes the ability to compare bundle sizes of any ref pushed to
Github. It is now only possible to compare the bundle sizes of
releases. This is because the [Origami Build Service](https://www.ft.com/__origami/service/build/) drops support
for bundling commit references for components which follow v2 of
the Origami Component Specification.
- Requires 3 arguments, the component name and two versions to compare.

closes: https://github.com/Financial-Times/origami-bundle-size-cli/issues/161